When such auxiliary speed change mechanism is fashioned to a mechanical one in a tractor having a transmission casing which is fastened to the rear of a clutch housing and includes a main speed change mechanism, a transmission assembly including the auxiliary speed change mechanism is usually fashioned as shown, for example, in JP, U No. 58-22557 and JP,U No. 59-37131, such that an input shaft for the transmission casing is disposed co-axially with a drive shaft of the driven side of the main clutch, and such that a counter shaft which is connected drivingly to the input shaft is disposed parallel to the drive and input shafts within a rear portion of the clutch housing. And, the auxiliary speed change mechanism is fashioned such that it performs a two-stage speed change transmission between the drive shaft and the input shaft through the counter shaft. When the counter shaft is removed from this structure and the input shaft for the transmission casing is directly coupled to the co-axial drive shaft, a transmission assembly is provided which is adapted for use in tractors of a design having no auxiliary speed change mechanism within the clutch housing. It is thus seen that the structure set forth above permits an easy conversion of transmission assemblies between tractors of a design having an auxiliary speed change mechanism within a clutch housing and of another design including no auxiliary speed change mechanism and will contribute to economy.
Of the two publications set forth above, JP, U No. 58-22557 discloses a transmission assembly in which an auxiliary speed change mechanism is supported by a bearing support plate, which is secured onto a front surface of an internal partition wall of the clutch housing so as to cover a large opening in the partition wall, and by a rear wall of the clutch housing. In the transmission assembly disclosed in JP, U No. 59-37131, an auxiliary speed change mechanism is housed in an auxiliary gear box which is disposed within a rear portion of the clutch housing and is secured onto a front surface of the transmission casing.
The reason why an internal partition wall is provided in the clutch housing or a separate auxiliary gear box is provided for housing a auxiliary speed change mechanism, as described above, is that an auxiliary speed change mechanism requires lubrication thereof so that it is necessary to interrupt lubricant oil for the change mechanism from a dry-type main clutch which is disposed within a front end portion of the clutch housing.
In a case where a rear wall of the clutch housing is used as a support member for the rearer side of an auxiliary speed change mechanism, as is the case in the transmission assembly shown in JP, U No. 58-22557, the auxiliary speed change mechanism is to be assembled necessarily within the clutch housing so that assembling procedure of the change mechanism is very troublesome. Contrary to this, a separate auxiliary gear box such as shown in JP, U No. 59-37131 permits an easy assembling procedure because the separate gear box housing in it an auxiliary speed change mechanism may firstly be secured onto a front surface of the transmission casing and, then, the clutch housing may be secured to the front of transmission casing. On the other hand, the separate gear box will enlarge weight of a transmission assembly and also will require a tight sealing structure for preventing leakage of lubricant oil into the clutch housing.
In a case where a bearing support plate to be secured to an internal partition wall of the clutch housing is used as a support member for the fronter side of an auxiliary speed change mechanism, as shown in JP, U No. 58-22557, it is required, when the transmission assembly is converted into the one of a type having no auxiliary speed change mechanism within the clutch housing, that a large opening in the partition wall is to be covered using a bearing support plate of a different design or structure. Further, in a tractor including an auxiliary speed change mechanism of fluid clutch-operated type, which has a large axial width as compared to a mechanically operated one, as shown, for example, in JP, B2 No. 63-39449, the internal partition wall of clutch housing which is located in accordance with the axial width of a mechanically operated change mechanism must be replaced into a fronter location. This means that a clutch housing of an entirely different design is required.
In the structure shown in JP, U 59-37131, the clutch housing does not include an internal partition wall. It is, however, preferred to provide an internal partition wall for securing rigidity of a clutch housing with keeping a small thickness of outer walls of the housing, as is done in many tractors.